Any American who pays attention to law enforcement has heard of the strategies: "broken windows," "stop and frisk," "zero tolerance." These are all variations on what's broadly known as "proactive policing": efforts to seek out and prevent the causes of crime before it happens, as opposed to a more reactive policy of just sending police when called.

A controversial alt-right political rally planned for San Francisco on Saturday afternoon has been canceled. Organizers announced the decision Friday afternoon, saying hostility from local politicians and leftist activists made the situation too dangerous. But concerns about protests on both sides linger, so the police remain on alert.

It's rare for a law enforcement officer to be convicted of homicide for shooting someone while on duty. According to a new NPR data analysis, 2,400 people have been killed this way in the last two and a half years; the vast majority of those cases were found to be justified, but NPR found 20 officers who faced charges. Of those, six have been convicted or pleaded guilty.